1. Field of the Invention
This invention relates to a combustion state-detecting system for internal combustion engines, and more particularly to a combustion state-detecting system which detects a state of combustion (misfiring) in the engine, based on a variation in the angular velocity of the crankshaft during rotation of the engine in every combustion cycle.
2. Prior Art
To detect a cylinder of an internal combustion engine in which normal firing does not take place due to failure in the ignition system, the fuel supply system or the like of the engine, an abnormality-detecting system is conventionally known, for example, by Japanese Provisional Patent Publication (Kokai) No. 3-286166, which detects a rate of variation in the angular velocity of the crankshaft during rotation of the engine in every combustion cycle, to thereby determine whether or not an abnormality has occurred in any of the cylinders, based on the detected rate of variation.
In the above conventional abnormality-detecting system, however, when a single or sporadic misfire has occurred in a particular cylinder, the engine undergoes so-called reactionary vibrations following the sporadic misfire to cause fluctuations in the angular velocity of the crankshaft. As a result, it is erroneously detected that a plurality of misfires have occurred, though actually only a single misfire has occurred.
To avoid such an erroneous detection caused by reactionary vibrations following a sporadic misfire, the present assignee has already proposed a combustion state-detecting system by Japanese Provisional Patent Publication (Kokai) No. 6-101560 and U.S. Ser. No. 08/120,832 corresponding thereto, which analyzes patterns of variation in the rotational speed of the engine which can be obtained after misfires, and inhibits a determination from being rendered that a misfire has occurred if the engine rotational speed varies in accordance with a pattern of variation in the engine rotational speed specific to reactionary vibrations following a misfire.
According to this proposed system, a rate of variation in the rotational speed of the engine is measured over a predetermined time period, an average value thereof is calculated, and then the absolute value of the difference between the rate of variation and the average value thereof is calculated, followed by sequentially cumulating the absolute values every predetermined time period, to thereby obtain a difference cumulative value. If the difference cumulative value exceeds a predetermined value, it is determined that the combustion state of the engine is degraded. Further, a rate of variation in the rotational speed of the engine over one-past combustion cycle (corresponding to consecutive generation of four TDC signal pulses if the engine is a four-cylinder engine) from the time the degradation of the combustion state is determined is analyzed. When a minimal value (or a maximal value) exists in the rate of variation in the rotational speed over the one past combination cycle, it is determined that a cylinder where spark ignition took place two TDC signal pulses before the cylinder corresponding to the minimal or maximal value misfired. Furthermore, to avoid erroneous detection caused by reactionary vibrations, a minimal value (or maximal value) obtained at a present TDC signal pulse, which can lead to a determination that a misfire has occurred, is compared with another minimal value (or maximal value) obtained three TDC signal pulses before the present TDC signal pulse, and if the former is larger (or smaller) than the latter, it is determined that the cylinder corresponding to the present TDC signal pulse or the present minimal (or maximal) value has not misfired.
In the above proposed system, however, to avoid erroneous detection caused by reactionary vibrations, a comparison is made between a minimal or maximal value of the rate of variation in the rotational speed obtained at the present TDC signal pulse and one obtained three TDC signal pulses before the present TDC signal pulse. In other words, a comparison is made between minimal or maximal values of the rate of an engine speed variation occurring in a predetermined pattern. As a result, minimal or maximal values of the rate of engine speed variation occurring in patterns other than the above pattern can all lead to determinations that misfires have occurred. Specifically, when a particular cylinder (e.g. #1 cylinder) undergoes intermittent sporadic misfires, for example, when the present cylinder is determined to have misfired due to a failure to supply fuel to the cylinder, even if fuel is properly injected for the following cylinder, part of the injected fuel adheres to the inner wall of the intake pipe of the engine, whereby a less amount of fuel than the amount required for combustion is supplied to the above following cylinder. Consequently, the air-fuel ratio of a mixture supplied to the cylinder becomes so lean that sufficient engine torque cannot be obtained, resulting in further increased reactionary vibrations, which causes-occurrence of a minimal or maximal value every combustion cycle (every four TDC signal pulses) in a pattern different from the above-mentioned three TDC signal pulse pattern. As a result, it can be determined that the engine has misfired, though actually it is in a normal combustion state.
Further, if to avoid erroneous detection caused by reactionary vibrations, a comparison is made, for example, between minimal or maximal values of the rate of variation in the rotational speed occurring within four TDC signal pulses, this is disadvantageous in the case where a particular cylinder continuously undergoes misfires (continuous misfiring), because a minimal or a maximal value of the rate of variation in the rotational speed occurs every four TDC signal pulses, and as a result, the engine can be erroneously, determined to be in a normal combustion state depending on the comparison result of the minimal or maximal values, though actually the engine is in a misfiring state.
Furthermore, in the case where minimal or maximal values of the engine speed variations caused by reactionary vibrations occur in patterns other than the four TDC signal pulse pattern, the above proposed system cannot perform determination of occurrence of misfires, based on such minimal or maximal values of the engine speed variation.